The present invention relates to an optical branching/multiplexing apparatus. More to particularly this invention relates to an optical branching/multiplexing apparatus which implements branching and multiplexing of wavelength in Wavelength Division Multiplexing transmission (WDM) system.
Description of the Prior Art
This kind of apparatus is called as an optical ADM (Add-Drop Modulator), and is shown in the Japanese Patent Application Laid-Open No. HEI 7-202299, or [GENERAL MEETING OF ELECTRONIC INFORMATION COMMUNICATION ASSOCIATION--1996 "Optical ADM Experiment and Limiting Content Therefor Using Fiber-Grating" pp, 747-748, March-1996 ] or the like.
FIG. 1 shows a system which implements wavelength division multiplexing transmission among local stations A, B, and C as a prior art example, wherein an optical ADM apparatus 8a is connected to among the local stations A, B, and C. The optical ADM apparatus 8a includes an optical branching device 1 and an optical multiplexing device 3. The optical branching device 1 causes a light signal with a plurality of wavelengths inputted as main input from an optical transmitter 16-1 of the local station A to branch selectively or to penetrate selectively, thus outputting a branch-signal to an optical receiver 15-2 of the local station C, and outputting penetrated signal to the optical multiplexing device 3 respectively. The optical multiplexing device 3 multiplexes the penetrated signal of the optical branching device 1 and the signal from the optical transmitter 16-2 of the local station C to output to the optical receiver 15-1 of the local station B as a main output.
However, in the above described conventional optical ADM apparatus 8a, when signal level of wavelength penetrated without branching by the optical branching device 1 differs from signal level of wavelength which is multiplexed, there is a problem that it brings harmful influence to transmission characteristic of wavelength which is penetrated.